Microstrip transmission lines are very popular in the radio frequency (RF) and microwave product design because of their lower cost and lower profile compared to the other transmission media such as the waveguides and striplines. Like the waveguides, a microstrip transmission line is a transmission medium that guides an Electro-Magnetic Wave (EM wave) between two points on a printed circuit board (PCB). FIG. 1 illustrates a traditional signal transmission model 100 in a PCB using a microstrip 102 having a width “W” sitting over a dielectric substrate 104 having a thickness “H” and a ground plane 106.
A microstrip transmission line is made up of a conducting strip placed over a dielectric substrate. The dielectric substrate comprises a ground plane placed below the substrate. The substrate in a PCB is commonly made up of FR-4. At lower frequencies, the FR-4 material is commonly employed as the dielectric substrate occupying the region between the conductive strip and the ground plane. The FR-4 material is much more cost effective as compared to the other high frequency substrates such as RT/Duroid® and hence FR4 material is much popular in the PCB product design. When using a PCB with FR4 material for high frequency applications, the conducting strips have to be wide enough to enable a high transfer rate. However, wide transmission lines will have to be tapered in proximity of the PCB die in order to allow physical connection with the die. When the FR-4 substrate is used at high frequencies, it results in a drastic change in the characteristic impedance (Z0) due to the tapering of the conducting strips. The change in the characteristic impedance (Z0) leads to high reflections, thereby restricting the conducting strip's bandwidth and data rate.
The current digital technology demands high data rates in the tune of Gigabits per second which means that interconnects (which include the connectors, the PCB transmission lines, and the chip bond wires) as well as the inputs-outputs (IOs) should have bandwidth wide enough to accommodate the Gigabit per second data transfer rates. Such high and wide bandwidth mandates that the characteristic impedance (Z0) be controlled to avoid signal reflections, resonance and radiations which result in signal transmission losses.
One traditional approach for manufacturing wideband employs the use of specialized and thus expensive Radio Frequency (RF) packages and PCBs using expensive high permittivity substrate materials such as ceramic, mica, alumina and RT/Duroid®. In such a case, the tapering of the microstrip is not required. These special materials have low dielectric loss and attenuation of the signal is also minimal. The downside is that these special materials have high cost and subsequently higher manufacturing cost of PCBs. Accordingly the final PCB structure becomes very costly rendering its use unpractical for today's low-cost, mass-produced, and throwaway type commercial electronics gadgets.
As illustrated in FIG. 2a, Another existing approach is to change the substrate's (204) thickness H1 by introducing a second conducting plane 208 in between the top conducting strip 202 and the original ground plane 206. The structural change in the substrate's height allows much narrower transmission lines 202, W2, in the vicinity of the die (chip) 212 thus providing the flexibility of a dense microstrip 200 layout. In this structural change approach, the two ground planes, 206 and 208, are stitched together using the multiple vias 210 consisting of electrical connections between the ground planes. The abrupt change in the substrate's height and the presence of vias 210 results in large amount of transmission losses due to the presence of resonances and radiation as illustrated in FIG. 2b. Hence the proposed technique did not receive industrial success.